Driving of a heat engine by its starter is usually controlled by the user of the vehicle, who releases the ignition key once the engine makes a characteristic sound. However, there is a current tendency to make motor engines more and more silent, and this tendency makes it increasingly difficult for the driver to detect the end of the starting operation. This in turn leads to the application of severe, and quite unnecessary, forces on the starter. A number of devices are already known for cutting off a motor vehicle starter when the engine has started satisfactorily, that is to say when the engine is sufficiently autonomous to attain its slow running mode by itself.
In particular, it has been proposed to control the cut-off of the starter by comparing the battery voltage with a reference voltage, the starter being cut off when the battery voltage reaches this reference voltage. One device which makes use of such a method of control was described in the introduction of French patent specification No. 2 626 417. In this connection, it is known that when a starter begins the starting operation, during which the starter is driving the engine, the voltage across the terminals of the battery undergoes a sharp diminution, due to the high current flowing through the starter. The battery voltage regains its initial level once the engine has been started.
However, such an arrangement is not satisfactory. In particular, the battery voltage can vary considerably over a period of time, in particular as a function of its state of maintenance, or as a function of ambient temperature. As a result, if the battery voltage is too low, it can happen that the starter will not be cut off immediately after the engine has been properly started, and in consequence the starter may be in operation for far too long, quite unnecessarily.
By contrast, if the battery voltage is too high, it can happen that the starter will be cut off too soon, i.e. well before the engine has properly started.